This invention relates to a process for producing polycrystalline silicon and, more particularly, to an improved, cost-effective process for producing polycrystalline silicon from silane and fumed silica from silicon tetrachloride.
In known commercial processes for producing polycrystalline silicon (also called polysilicon) from silane (SiH.sub.4) , a major cost element is the cost of producing silane by either of two commercial production processes, i.e. reduction of silicon tetrafluoride by sodium aluminum hydride (NaAlH.sub.4) or redistribution of trichlorosilane produced by high pressure hydrochlorination of metallic silicon and silicon tetrachloride. In the latter process, as practiced commercially, the first step involves the hydrochlorination of silicon tetrachloride in a fluidized bed of silicon to produce the trichlorosilane as the starting material employed in the redistribution reactions. This hydrochlorination reaction carried out at a temperature of 500-550.degree. C. and a pressure of 500 psig is the most costly step of the commercial process for making silane. The trichlorosilane is converted by redistribution to silicon tetrachloride and dichlorosilane which in turn is converted by redistribution to silicon tetrachloride and silane. The silane is then subjected to pyrolysis in either a fixed rod type or fluidized bed reactor to produce polycrystalline silicon and hydrogen. The overall process may be summarized as follows:
1. Hydrochlorination: 3SiCl.sub.4 +2H.sub.2 +Si.fwdarw.4 HSiCl3 PA1 2. Redistribution: 2HSiCl.sub.3 .fwdarw.SiCl.sub.4 +H.sub.2 SiCl.sub.2 PA1 3. Redistribution: 2H.sub.2 SiCl.sub.2 .fwdarw.SiCl.sub.4 +SiH.sub.4 PA1 4. Pyrolysis: SiH.sub.4 .fwdarw.2H.sub.2 +Si PA1 Trichlorosilane Synthesis: EQU 4 HCl+Si.fwdarw.HSiCl.sub.3 (88%)+SiCl.sub.4 (10%)+H.sub.2 PA1 Redistribution Reactions: EQU 4 HSiCl.sub.3 .fwdarw.3 SiCl.sub.4 +SiH.sub.4 PA1 Pyrolysis: EQU SiH.sub.4 .fwdarw.2H.sub.2 +Si PA1 a) subjecting impure silicon to hydrochlorination with hydrogen chloride to produce trichlorosilane and silicon tetrachloride together with minor amounts of dichlorosilane and monochlorosilane; PA1 b) converting the trichlorosilane to silicon tetrachloride and silane; PA1 c) converting the silane to polycrystalline silicon and hydrogen; PA1 d) reacting the silicon tetrachloride from steps a) and b) with hydrogen and oxygen to produce fumed silica and hydrogen chloride; and PA1 e) recycling the hydrogen chloride from step d) for use in step a).
Equations 1, 2 and 3 represent partial reactions. If one recycles all the unreacted starting materials and by-products, a closed loop process is achieved with the overall, net result being the conversion of metallurgical silicon from equation 1 to polycrystalline silicon in equation 4. Thus, the hydrogen and chlorine are merely carriers and, after the initial charge represented by equation 1, only makeup hydrogen and silicon tetrachloride are needed to replenish the processing loss of these two reactants.
It is also known to produce silane by the reaction of metallurgical or impure silicon and hydrogen chloride in a fluidized bed reactor at a temperature of about 300 to 400.degree. C. The reaction produces a high yield (approximately 88%) of trichlorosilane, approximately 10-12% of silicon tetrachloride and minor amounts of dichlorosilane and monochlorosilane. The trichlorosilane is used to make dichlorosilane and silicon tetrachloride and the dichlorosilane produced is further redistributed to make silane as in the previous reaction scheme shown above. The silane is then used to make polycrystalline silicon by pyrolysis. This sequence of reactions may be represented by the following simplified equations:
This process of producing silane has heretofore not been regarded as a practical commercial process because of the large amount of silicon tetrachloride produced as a by-product even though the hydrochlorination of silicon with HCl to make trichlorosilane can produce the latter at a lower cost than that of the hydrogenation process previously outlined above.
It is also known to produce fumed silica by burning silicon tetrachloride and hydrogen with oxygen in a burner. This combustion process produces fumed silica and hydrogen chloride (HCl).
There is a continuing need to develop more cost efficient processes for producing polycrystalline silicon from silane and fumed silica from silicon tetrachloride.